Tissue organization requires the formation of specific intercellular junctions in order to create three-dimensional structures in organs and to define internal and external fluid compartments. One type of intercellular junction is known as the adherens junction, which cements neighboring cells together.

Vasioukhin et al. have examined the dynamic aspects of junction formation in freshly isolated, primary cultures of epithelial cells and find that the production of junctions is a complex phenomenon involving several morphological stages. First, in response to increased extracellular calcium, each cell extrudes actin-rich projections known as filopodia; neighboring cells are drawn into close contact by these interdigitating filopodia. Then, adherens junction proteins cluster at the tips of the filopodia and serve to button cells together at these contact points. Finally, the actin cytoskeleton is reorganized to zip up the membranes of two cells along the surfaces between adherens junctions. This process provides the driving force to push the cells together tightly to form the junctional seal.

Raich et al. have used time-lapse, multiphoton laser-scanning microscopy to visualize these events—extension of filopodia, recruitment of junctional proteins, and sealing of the epithelial sheet—during embryonic development of Caenorhabditis elegans.—SMH